1. Field of Invention
This invention pertains to an industrial structure constructed to meet cleanroom specifications such as are required for the production of integrated circuit chips, electronic components and other products which are subject to contamination in an environment of dust or other forms of microcontamination. More particularly, the present invention relates to modular construction for a cleanroom wherein the enclosing structure can be assembled or disassembled in components, and wherein the assembled cleanroom structure can be modified by removal of structural elements without affecting the cleanroom certification for remaining, unchanged structure.
2. Prior Art
The advance of modern technology into the world of miniaturization has necessitated the development of unique manufacturing environments. For example, the preparation of a multilayered, integrated computer chip includes the fabrication of wafer masks and layouts that include hundreds of tiny circuits whose operational condition depends upon the absence of foreign materials. These chips are prepared in an industrial cleanroom which is classified based upon the amount of microcontamination within the room. A Class 100 room at 0.5 microns means that a cubic foot of controlled airspace within the cleanroom will have no more than an average total of 100 particles no larger in size than 0.5 microns. Likewise, a Class 10 room means that there are only 10 or less particles of 0.5 microns in size or larger in a cubic foot of airspace.
The trends for increased productivity and chip capacity have increased the need for more stringent standards for cleanroom structures. Movements have developed to reduce the standard of measurement from 0.5 microns to 0.2 microns and to anticipate Class 1 cleanroom conditions. Research efforts to reach these goals have focused on development of filter systems which operate to remove microcontamination greater than the prescribed size from circulating air. Flanders Filters, Inc., for example, has developed HEPA filters which establish a Class 10 condition based on 0.1 micron particle size. More controlled environments are being realized with a cleanroom by setting up individual hooded work stations where increased filter action is applied to a greatly reduced workspace of several cubic yards. Efforts are also being made to improve the airflow pattern to reduce turbulence, which tends to accumulate pockets of dust and other forms of contamination.
The realization of improved cleanroom requires special attention to room construction as well as air filter systems. As the Class 1 condition is approached, focus must extend to both filter system and room-tight construction features. Because of the need for rigid, air tight construction, typical cleanroom facilities are permanent structures. Structural modifications to change wall position, room access or design are difficult and sometimes impractical because other joints and seams at wall junctions, windows, doors, ceiling, etc., may be affected. Shifting a segment of wall may cause a slight movement in another wall section. This slight movement may be undetectable to the eye; however, it could be a wide-open door to microcontamination. This results in extended down-time and certification expense.
Although permenant structures were acceptable for earlier cleanrooms, modern industry demands greater structural adaptability. Many of these demands are not only a natural outgrowth of the changing technology in wafer production, but also include longstanding design problems unique to cleanroom production. For example, manufacturing equipment used within the cleanroom may require day to day maintenance. To protect the cleanroom environment, the equipment is typically installed as part of a service wall with the operational side sealed to the interior of the cleanroom and the maintenance/access side exposed to a service aisle adajacent the cleanroom. Although repair work is facilitated by the service wall configuration, removal of machinery may involve modification to wall structure. Therefore, if equipment must be replaced, risk of damage to the cleanroom structure exists. Equally challenging is the need for an adapted wall structure which allows modification in configuration to permit change to new equipment as the industry progresses. With present facilities such progressive changes in equipment may require construction of a new cleanroom facility, particularly where major remodeling of the loadbearing wall structure is required. Such structural limitations illustrate the distinction of cleanroom structures from facilities not subject to such severe limitations.
Further uniqueness of the cleanroom as a separate field of art as compared to general construction is evident in the modern trend toward use of overhead plenum/filter air purification systems. The plenum is usually constructed in an overhead position with a full suspended ceiling utilizing a HEPA filter system. Large airconditioning units are supported over the plenum structure and force the air into the plenum volume. This plenum must be a fully sealed, rigid structure to preserve strict cleanroom conditions. Attempts to remove wall structure below this heavy plenum for replacement of equipment can not only breach the cleanroom condition of the wall enclosure, but can also compromise the certification of the plenum as well.
The financial impact of a nonoperational or defective cleanroom is staggering. For example, the production cost for a sheet of wafers or computer chips is the same, regardless of the percent of wafers that are acceptable. If only 10% of the batch are usable because 90% are contaminated due to loss of cleanroom condition, the production cost can far exceed potential sales receipts. In a premium cleanroom environment, however, the same production cost can develop a 50 to 75% yield. Economical productivity in the wafer industry is therefore a direct function of cleanroom serviceability.
Because of the high risk and critical impact of microcontamination in chip manufacturing, prior art cleanroom structures have been fabricated in accordance with permenant design features, as opposed to temporary or modular construction. Specific parts of walls are either designed for permanent loadbearing function or service wall access and are not adapted to be interchangable. The development of a portable, modular cleanroom enclosure has been deemed impractical for meeting the stringent needs of future class 1 structural requirements.